This invention relates generally to centrifugal pumps and more particularly to centrifugal pumps having intermittent dry running capability provided to a product lubricated bearing or to mechanical seal(s) by a fluid percolation system.
At high rotating speeds at which centrifugal pumps operate, product lubricated bearings or mechanical seal(s) (henceforth bearings and seals) must be lubricated at all times to reduce friction and to carry away heat generated by the friction which persists. With perfect lubrication, there would be no friction and, consequently, no heat build-up to be carried away. Since many fluids, such as water have limited lubricity, and since those same fluids often dissolve lubricating oils and greases, it is fairly common to use the pumped (or working) fluid to lubricate the bearings and seals of a centrifugal pump. Because of the limited lubricity of many pumped fluids, it is common to use very hard materials in pumps lubricated by the pumped fluid. This arrangement is generally quite satisfactory, except that occasionally the fluid (or liquid) source becomes empty while pumping, and the pump runs dry for a short time. This can lead to a very rapid temperature rise due to frictional heating and can destroy bearings and seals in a short time.
Product lubricated pump bearings and seals typically cannot be safely operated under dry system conditions; because the materials from which they are made are not sufficiently lubricious and because they are not able to withstand the temperatures generated by such operation. Bearings and seal surfaces are often made of silicon carbide, because it is hard and chemically resistant and is therefore tolerant of suspended particles and corrosive liquids; however, silicon carbide fails almost instantaneously without lubrication. To provide for occasional dry running, bearings have been made from carbon and/or graphite, which can withstand approximately 3-5 minutes of dry running; but these materials are soft and wear rapidly when pumping fluids which contain particles. Various grades of silicon carbide with graphite, polymers, or other materials have been offered, but none has a dry run capability of more than 5-10 minutes. Similarly, the sealing surfaces of mechanical seals require continuous vapor flow across the seal faces to prevent failure which could result from dry running.
The foregoing illustrates limitations known to exist in present centrifugal pumps. Thus, it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.